带有拒绝调用和抛出错误的Promise构造函数

Promise constructor with reject call vs throwing error

本文关键字：错误 Promise 构造函数 出错 拒绝 调用      更新时间：2024-04-20

在以下代码中：

var p1 = new Promise(function (resolve, reject) {
    throw 'test1';
});
var p2 = new Promise(function (resolve, reject) {
    reject('test2');
});
p1.catch(function (err) {
    console.log(err); // test1
});
p2.catch(function (err) {
    console.log(err); // test2
});

从Promise api使用reject（在p2中）和使用throw抛出错误（在p1中）之间有什么区别吗？

完全一样吗？

如果是一样的，为什么我们需要reject回调？

从Promise api使用reject（在p2中）和使用throw抛出错误（在p1中）之间有什么区别吗？

是的，不能异步使用throw，而reject是一个回调。例如，一些超时：

new Promise(_, reject) {
    setTimeout(reject, 1000);
});

完全一样吗？

不，至少当其他代码跟随您的语句时不会。throw立即完成解析器函数，而调用reject则在将promise"标记"为已拒绝后继续正常执行。

此外，如果throw错误对象，引擎可能会提供不同的异常调试信息。

对于您的具体示例，p1和p2与外部无法区分是正确的。

我知道这有点晚了，但我真的不认为这两个答案都能完全回答我发现这个问题时的问题，这里有一个更完整的例子。

var p1 = new Promise(function (resolve, reject) {
    throw 'test 1.1'; //This actually happens
    console.log('test 1.1.1'); //This never happens
    reject('test 1.2'); //This never happens because throwing an error already rejected the promise
    console.log('test 1.3'); //This never happens
});
var p2 = new Promise(function (resolve, reject) {
    reject('test 2.1'); //This actually happens
    console.log('test 2.1.1'); //This happens BEFORE the Promise is rejected because reject() is a callback
    throw 'test 2.2'; //This error is caught and ignored by the Promise
    console.log('test 2.3'); //This never happens
});
var p3 = new Promise(function (resolve, reject) {
    setTimeout(function() { reject('test 3.1');}, 1000); //This never happens because throwing an error already rejected the promise
    throw('test 3.2'); //This actually happens
    console.log('test 3.3'); //This never happens
});
var p4 = new Promise(function (resolve, reject) {
    throw('test 4.1'); //This actually happens
    setTimeout(function() { reject('test 4.2');}, 1000); //This never happens because throwing an error already rejected the promise
    console.log('test 4.3'); //This never happens
});
var p5 = new Promise(function (resolve, reject) {
    setTimeout(function() { throw('test 5.1');}, 1000); //This throws an Uncaught Error Exception
    reject('test 5.2'); //This actually happens
    console.log('test 5.3'); //This happens BEFORE the Promise is rejected because reject() is a callback
});
var p6 = new Promise(function (resolve, reject) {
    reject('test 6.1'); //This actually happens
    setTimeout(function() { throw('test 6.2');}, 1000); //This throws an Uncaught Error Exception
    console.log('test 6.3'); //This happens BEFORE the Promise is rejected because reject() is a callback
});
p1.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test1
});
p2.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test2
});
p3.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test3
});
p4.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test4
});
p5.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test5
});
p6.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test6
});

不，没有，两者完全相同。我们需要reject的唯一区别和原因是当您需要异步拒绝时——例如，如果您正在转换基于回调的API，它可能需要发出异步错误信号。 
var p = new Promise(function(resolve, reject){
    someCallbackApi(function(err, data){
        if(err) reject(err); // CAN'T THROW HERE, non promise context, async.
        else resolve(data);
    });
});

一个非常有趣的观察结果是，如果使用throw，它将首先由reject处理程序&如果拒绝处理程序未就位，则CCD_ 22处理程序。

带有拒绝处理程序块

var allowed = false;
var p1 = new Promise(
function(resolve, reject) {
  if (allowed)
    resolve('Success');
  else
//         reject('Not allowed');
    throw new Error('I threw an error')
})
p1.then(function(fulfilled) {
console.log('Inside resolve handler, resolved value: ' + fulfilled);
}, function(rejected) {
console.log('Inside reject handler, rejected value: ' + rejected);
}).catch(function(error) {
console.log('Inside error handler, error value: ' + error);
})

没有拒绝处理程序块

var allowed = false;
var p1 = new Promise(
function(resolve, reject) {
  if (allowed)
    resolve('Success');
  else
//         reject('Not allowed');
    throw new Error('I threw an error')
})
p1.then(function(fulfilled) {
console.log('Inside resolve handler, resolved value: ' + fulfilled);
}).catch(function(error) {
console.log('Inside error handler, error value: ' + error);
})

此外，catch块将能够捕获resolve处理程序内部抛出的任何错误。

var allowed = true;
var p1 = new Promise(
function(resolve, reject) {
  if (allowed)
    resolve('Success');
  else
//         reject('Not allowed');
    throw new Error('I threw an error')
})
p1.then(function(fulfilled) {
console.log('Inside resolve handler, resolved value: ' + fulfilled);
throw new Error('Error created inside resolve handler block');
}).catch(function(error) {
console.log('Inside error handler, error value: ' + error);
})

看起来最好使用throw，除非你不能。如果你正在运行一些异步任务，你将不得不将reject回调传递给异步函数。但还有一个解决办法，那就是承诺您的异步函数。上的更多信息https://stackoverflow.com/a/33446005

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